Building Costs in 1999 please.

[LANDLADY43]

Hello - can anyone give me an idea of building costs for 80 sq m (860sqft) two-story extension in 1999 or thereabouts in the Southeast of England?    We built an extension to our house 25 years ago and cannot find the invoices and need a rough estimate please for Capital Gains Tax purposes.

Many thanks for your assistance,

[Temp]

I asked ChatGPT..

 

Quote

Can you estimate costs for building a 80sqm two storey residential extension in southeast England in 1999. Answer in in £/sqm and £/sqf

 

It responded..

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For a two-storey residential extension in southeast England in 1999, the cost per square metre was typically lower per floor than a single-storey extension, because the foundation and roof costs are shared over more floor area.

 

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🔹 Estimated 1999 Costs for Two-Storey Extension

 

💷 Cost per square metre (£/sqm)

 

£600 – £1,000 per sqm (for total floor area, i.e., both storeys)

📐 Cost per square foot (£/sqft)

 

1 sqm = 10.764 sqft

 

So:

£600/sqm ≈ £55.74/sqft

£1,000/sqm ≈ £92.90/sqft

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🏠 For an 80 sqm Extension (i.e., 40 sqm per storey):

 

Low estimate: 80 sqm × £600 = £48,000

High estimate: 80 sqm × £1,000 = £80,000

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Notes:

These figures assume standard construction and finishes.

The 80 sqm total means two floors of 40 sqm each.

Southeast England, especially near London, tends toward the higher end of the range.

Additional costs (kitchens, bathrooms, structural alterations) would be extra.

 

Would you like this adjusted for 2025 prices or compared with current market rates?

 

 

I'm afraid I've no idea how accurate this is.

 

The graph here...

https://costmodelling.com/construction-indices

 

...suggests build costs have gone up about 2.5 times from 2000 to 2025. That would put the cost today at between £1500 and £2500 per sqm today which I think is about right.

 

An awful lot depends on the complexity of the extension. There could be quite a difference between a simple two storey extension and a wrap around with lots of steel to make it open plan with the main house.

[nod]

If it’s a rental They will want receipts

If not Think of a number and double it 

[LANDLADY43]

Already a great help.   It was our residential home, so hope Nod is right.  Thanks also to Temp, great information.